1. Field of the Invention
The present invention relates to a cable sheath having an intumescent composition that contains no halogen, and it relates in particular to a flame-retardant cable sheath, in particular for a telecommunications cable. The invention also relates to a cable whose properties, in particular its flame-retardant properties, are significantly improved compared with currently known cables.
2. Description of the Related Art
Cable, in particular cable used for telecommunications is made up of one or more cores surrounded by a protective covering made up of one or more layers. The core can be a metal core or an optical-fiber core. The covering surrounding the core is made up of layers having various properties, e.g. insulating properties or water-repellent properties. When the cable is made up of a plurality of individually covered cores, they are brought together to form a bundle. The bundle is also surrounded by a sheath in which at least one layer (usually the outer layer) is made from a composition imparting flame-retardant properties to it.
Materials used for cable sheath have to satisfy certain requirements. They must not be pollutant or toxic. They must offer good fire-resistance and ignition retardancy. Their combustion must not give rise to high smoke emission. It is also desirable for the smoke given off to be of low opacity, of low toxicity, and of low corrosiveness. In addition sheathing materials must offer good resistance to deformation when they are exposed to heat. In particular when the materials melt under the effect of the heat, the drops formed as they melt give rise to risks of causing burns, and they can also cause the fire to spread by generating new starting points from which fire can break out.
To form such a layer, compositions are known that are based on polymers and that contain flame-retardant additives. The current trend is to seek to replace polymer compositions that include halogen-containing additives and that have hitherto been used for cable insulation sheathing. Standards are tending to prohibit the use of such additives not only because of the toxicity and corrosiveness of the products given off when they burn, but also because they constitute possible health hazards while they are being manufactured and while they are being incinerated. In addition, the current trend is to give preference to implementing substances that are easily degradable or even recyclable. Such materials for retarding propagation of flames and fire and that do not contain halogens are often referred to as “halogen-free fire retardants” (HFFRs).
One known solution for making polymer cable sheaths more fire-resistant consists in adding a filler constituted by a metal hydroxide such as Al(OH)3 or Mg(OH)2. Unfortunately, it is necessary to add a considerable quantity of filler in order to obtain sufficient protection from flames. It is then observed that the mechanical and electrical properties of the material are degraded. Furthermore, such compositions have high viscosity, and they are therefore difficult to implement, in particular by extrusion.
Compositions are also known that contain associations of silicone oil, of calcium carbonate, and of magnesium stearate. However, the performance of such compositions still leaves room for improvement.
Additives are known for causing intumescence, i.e. swelling, of a polymer composition, but they suffer from drawbacks. Thus, pentaerythritol, which is often used, starts to decompose at 200° C., which might pose problems during extrusion. Melamine, also used as a char-forming agent, significantly increases the concentration of cyanide ions in the degradation products.
Document EP-1 026 700 discloses a cable sheath made of a halogen-free material composed of a polymer, of an intumescent component which causes the material to swell, and of an inorganic glass former which forms a flame-resistant inorganic foam. That cross-linkable composition also includes silanes or peroxides as cross-linking agents. The extruded sheaths are cross-linked statically in a separate step, after the cable has been extruded. That method results in full cross-linking, and in a material that is difficult to recycle.